cpu: rnn: enable f16 for non-amx isa

Author: ankalinin

src/cpu/x64/rnn/rnn_brgemm_utils.cpp

@@ -89,7 +89,7 @@ x64::cpu_isa_t brgemm_calc_isa(
     } else if (rnn.is_cell_dt_bf16()) {
         return x64::avx512_core_bf16;
     } else if (rnn.is_cell_dt_f16()) {
-        return isa_undef;
+        return x64::avx512_core_fp16;
     } else { // f32
         return utils::map(true, x64::isa_undef, mayiuse(avx512_core),
                 avx512_core, mayiuse(avx2), avx2);